The present invention relates to portable resistance spot welding guns, called "transguns," and, more particularly, to a safety circuit for such transguns. Historically transguns have been used with robots, but recently a number of applications throughout Europe and North America have made use of a human operator handling the transgun and making welds by initiation of the welding sequence after placing the transgun in a welding position. Because transguns supply high voltages during the welding cycle in the vicinity of the operator, safety circuitry is required in the transgun to protect the operator from electric shock.
A typical transgun circuit 10 is shown in FIG. 1. 480 or 575 volt AC (Canada) power 12 is supplied to a welding control enclosure 14. Circuit breaker 15, isolation contactor 17 having two contacts and silicon control rectifiers (SCRs) 19 are all contained within welding control enclosure 14. Welding control enclosure 14 is connected to the transgun 16 via welding cable 18. Transgun 16 comprises transformer 25, electrodes 22, and material to be joined 20. The transgun is manipulated into position over the sheets to be welded 20, and depressing a button on the transgun initiates the welding action.
FIG. 2 shows the shock hazard that may be caused by wear on the welding cable of the transgun. All of the following conditions must exist for such a hazard to occur:
1. The power supply system must have a ground fault. This is shown in FIG. 2 as one leg of the three phase delta connected system being grounded (26). PA1 2. The weld control must be powered up to close the circuit breaker (circuit breaker 15 closed). PA1 3. The weld control must be initiated to close the isolation contactor 17. PA1 4. The operator 28 must contact one of the welding cable's "live" conductors via worn spot 31. PA1 5. The operator must be grounded (30).
If all of the above conditions are met, fault current 32 will flow (as shown in FIG. 2) from one leg of the power supply through the closed circuit breaker, the closed isolation contractor, the welding cable up to the point of the fault, through the fault to the operator, through the operator to ground, and finally back to the ground at the delta power system. It should be noted that in grounded "Y" power systems there is no need for a fault condition to ground one leg of the distribution system, since the grounded "Y" is intentionally grounded. Therefore, for grounded "Y" systems, only the last four conditions must be met to provide the possibility of shock to the operator.
FIG. 3 shows a timing diagram for the fault current shown in FIG. 2. The timing diagram shows that after operator initiation of the welding cycle, the welder continues through the entire welding sequence, even though fault current is flowing. The worst case fault current is calculated by assuming that an operator's resistance to current flow is approximately 1000 ohms, so that with a supply voltage of 575 volts, approximately 575 milliamperes (ma) of current will flow for a time that is only limited by the weld time adjusted in the weld controller. This time interval could be as long as several seconds.
To prevent the situation described by FIG. 2 and FIG. 3, systems that incorporate ground fault current detection have been designed. A typical such system is shown in FIG. 4. These devices operate on the principle of current imbalance, and current imbalance detector 34 checks to insure that the current that is supplied to the transgun via one of the supply wires matches the current returning via the other wire. Any difference between the two currents measured is assumed to be due to a fault in the system, and if the level is high enough, circuitry operates the shunt trip coil of the circuit breaker, thus opening the circuit breaker and disconnecting the welding control, welding cable, and transgun from the supply voltage.
The timing diagram of FIG. 5 shows that although this approach does not limit the current that the operator is exposed to, it does limit the time before the high voltage is removed. FIG. 5 shows that the same 575 ma current will flow, but because the ground fault detector 34 ultimately causes the circuit breaker to open within 90 milliseconds (ms), the operator's exposure to the high voltage is reduced.
FIG. 6 shows curves defining "safe" operating conditions. Because there is not universal agreement on shock hazard, various organizations have adopted guidelines. The UL Class A curve is the most difficult to meet, because the current levels allowable are lower. Note that the worst case condition, 575 ma at 90 ms (36), is to the right of all the curves, while the preferred operating side is the left side of the curves.